Theses: Coherent and Non Coherent UWB Communications
J. Lopez-Salcedo

Abstract

Short-range wireless communication has become an essential part of everyday life thanks to the enormous growth in the deployment of wireless local and personal area networks. However, traditional wireless technology cannot meet the requirements of upcoming wireless services that demand high-data rates to operate. This issue has motivated an unprecedented resurgence of ultra-wideband (UWB) technology, a transmission technique that is based on the emission of sub-nanosecond pulses with a very low transmitted power. Because of the particular characteristics of UWB signals, very high data rates can be provided with multipath immunity and high penetration capabilities. Nevertheless, formidable challenges must be faced in order to fulfill the expectations of UWB technology. One of the most important challenges is to cope with the overwhelming distortion introduced by the intricate propagation physics of UWB signals. In addition to this, UWB antennas behave like direction-sensitive filters such that the signal driving the transmit antenna, the electric far field, and the signal across the receiver load, they all may differ considerably in wave shape. As a result, the well-known and popular concept of matched filter correlation cannot be implemented unless high computational complexity is available for perfect waveform estimation.

The lack of knowledge about the received waveform leads UWB receivers to be implemented under a coherent or non-coherent approach depending on a tradeoff between waveform estimation complexity and system performance. On the one hand, coherent receivers provide the reference benchmark in the sense that they have ideally perfect knowledge of the end-to-end channel response and thus, optimal performance is achieved. On the other hand, non-coherent receivers appear as a low-cost and low-power consumption alternative since channel estimation is not considered and thus, suboptimal performance is expected.

In the present dissertation, both coherent and non-coherent receivers for UWB communications are analyzed from a twofold perspective. In the first part of the thesis, an information theoretic approach is adopted to understand the implications of coherent and non-coherent reception. This is done by analyzing the achievable data rates for which reliable communication is possible in the presence and in the absence of channel state information. The simulation results show a different behavior when evaluating spectral efficiency of coherent and non-coherent receivers under the wideband regime. For this reason, and in order to shed some light on this issue, closed-form expressions are derived to allow an analytical study of the asymptotic behavior of constellation-constrained capacity.

In the second part of the thesis, the emphasis is placed on the design of signal processing techniques for carrying out the basic tasks of an UWB receiver. For the case of coherent receivers, a maximum likelihood waveform estimation technique is proposed based on the exploitation of second-order statistics. One of the key features of the proposed technique is that it establishes a clear link between maximum likelihood waveform estimation and correlation matching techniques. Moreover, the proposed method can be understood as a principal component analysis that compresses the likelihood function with information regarding the signal subspace of the received signal. As a result, significant reduction in the computational burden is obtained through a tradeoff between bias and variance.

For the case of non-coherent receivers, the problems of symbol detection and signal synchronization are addressed from a waveform-independent perspective. The framework of waveform-independent symbol detection is derived for the case of correlated and uncorrelated scenarios, and low-complexity implementations are proposed based on the maximization of the Jeffreys divergence measure. As for the synchronization problem, a nondata-aided and waveform-independent technique is proposed for the frame-timing acquisition of the received signal. Next, a low-complexity implementation is proposed based on the multifamily likelihood ratio testing by understanding the frame-timing acquisition of UWB signals as a problem of model order detection.


Slides




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Signal Processing and Communications group
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